1. Field of the Invention
This invention relates to radiation sensitive compositions such as photoresists that provide high solubility differentials between exposed and unexposed regions, and increased transparency to activating radiation.
2. Background Art
Photoresists are photosensitive films used for transfer of an image to a substrate. They may be negative or positive acting. After a coating of a photoresist is formed on a substrate, the coating is selectively exposed through a photomask to a source of activating energy such as ultraviolet light. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. The pattern in the photomask of opaque and transparent areas define a desired image to be transferred to a substrate.
In the case of a negative photoresist, exposed portions of a photoresist coating become less soluble in a developer as the result of a photochemical reaction, thereby resulting in differential solubility between the exposed and unexposed portions. This difference in solubility allows for the selective removal of unexposed portions of the photoresist coating and the subsequent transfer of an image to a substrate.
In the case of a positive acting photoresist, exposed portions of the photoresist coating become more soluble in developer than unexposed portions as the result of a photochemical reaction allowing for selective removal of exposed areas by development.
Following development of a photoresist coating, portions of the substrate bared by development may be altered such as by etching. The historical background, types and processing of conventional photoresists are described by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York, Chapter 2, 1975 and by Moreau, Semiconductor Lithography, Principles, Practices and Materials, Plenum Press, New York, Chapters 2 and 4, 1988, both incorporated herein for their teaching of photoresist compositions and methods of making and using the same.
Most commercial photoresist formulations, both positive and negative, comprise a film forming binder and a radiation sensitive component. Many of these film forming binders are phenolic resins. For example, many positive acting photoresists currently in commercial use comprise a novolak resin and a naphthoquinone diazide sulfonic acid ester photoactive compound where the novolak resin is the reaction product of formaldehyde and a phenol. Examples of such photoresists are disclosed in U.S. Pat. Nos. 4,377,631 and 4,404,272, incorporated herein by reference. Another class of positive acting photoresists comprise a poly(vinylphenol) and a naphthoquinone diazide sulfonic acid ester. Examples of these photoresists are disclosed in U.S. Pat. Nos. 3,869,292 and 4,439,516, both incorporated herein by reference.
An important property of a photoresist is image resolution. A developed image of fine line definition having vertical side-walls is highly desired to permit transfer of the fine line image to an underlying substrate.
Another important property of a photoresist is photospeed. Photospeed is a common way of describing the sensitivity of a photoresist or other radiation sensitive compositions to activating radiation. Enhanced photospeed is especially important in applications where a number of exposures are needed, for example, in generating multiple patterns by a step and repeat process, or where activating energy of reduced intensity is employed. Increased photospeed also permits reduction in the radiation sensitive component of the photoresist and/or a decrease in the required energy of exposure for image formation.
Some cationic photoinitiators have been used to induce selective photogenerated acidic cleavage of certain "blocking" groups pendant from a photoresist binder, or cleavage of certain blocking groups that comprise a photoresist binder. See, for example, U.S. Pat. Nos. 4,968,581; 4,883,740; 4,810,613 and 4,491,628, and Canadian Patent Application 2,001,384, all of which are incorporated herein by reference for their teaching of the described binders and acid labile blocking groups, and methods of making and using the same. Such cleavage is reported to create different solubility characteristics in exposed and unexposed areas of the polymer. Upon selective cleavage of the blocking group through exposure of the photoresist, a polar functional group is said to be provided, for example, carboxyl or imide.
It has been found that these reported systems have limitations. For example, to realize significant solubility differentials between exposed and unexposed regions (and thereby provide high resolution of the developed image), known systems generally require that a somewhat large portion of the photoresist binder contain acid labile blocking groups. That is, a large portion of the polar functionalities of the resist are substituted with blocking groups so that a sufficient solubility differential is provided between exposed and unexposed regions of the resist. Consequently, exposure results in cleavage of a significant mass of the resist. This can result in shrinkage of the photoresist in exposed regions and thereby compromise resolution of the image patterned in the photoresist coating layer.
Other problems can arise upon subsequent etching of the substrate that underlies the developed photoresist image. For example, to etch an aluminum or silicon oxide substrate rather stringent conditions are often employed. Aluminum chlorine gas is frequently used to etch such substrates and extensive localized heating often occurs during the etching sequence. As a consequence, the patterned photoresist coating on the substrate can experience shrinkage as the acid labile groups of the unexposed resist pattern undergo thermally induced cleavage. This can result in the imaged photoresist lines having a wrinkled or roughened surface, a condition known in the art as reticulation and which can be undesirable, particularly for high resolution applications.
In U.S. Pat. No. 5,128,232, incorporated herein by reference, a photoresist composition is described, the composition having a resin binder comprising a polymer having a major portion of phenolic units and a minor portion of cyclic alcohol units. This application discloses that by employing cyclic alcohol units in the binder, the resist composition exhibits enhanced transparency to activating radiation. This application also discloses that the concentration of the cyclic alcohol units should be limited to retain developer solubility of an exposed layer of the resist composition.
It would be desirable to have a photoresist binder that could provide high solubility differentials upon exposure with only a moderate substitution of the binder with acid labile groups. It also would be desirable to have a photoresist binder that contained acid labile groups but was resistant to reticulation during stringent processing steps. It would be further desirable to have a radiation sensitive composition that was highly transparent to activating radiation.